To improve system performance, radio frequency channel (RFCH) hopping, referred to herein as frequency hopping, is often employed in cellular, radio telecommunications systems, such as the Global System for Mobile Communication (GSM). In general, frequency hopping improves system performance by introducing frequency diversity and interference diversity, as will be explained in detail below. Frequency hopping is a well-known technique.
Frequency diversity is achieved by transmitting and receiving each radio telecommunications signal on a sequence of frequencies over time. Each signal is transmitted and received over a sequence of frequencies because radio signals are often subject to amplitude variations called Rayleigh fading. However, Rayleigh fading at a specific geographic location typically affects radio signals carried on certain frequencies more so than other frequencies. Thus, transmitting and receiving a radio telecommunications signal over a sequence of different frequencies increases the likelihood that the signal will be received correctly, as it is unlikely that Rayleigh fading will significantly impact each and every frequency over which the radio telecommunications signal is being transmitted. Accordingly, signal quality is improved and overall system performance is enhanced.
On the other hand, Interference diversity works as follows. In addition to fading, a radio signal is often subject to varying degrees of interference caused by traffic on the same frequency channel (i.e., co-channel interference) and traffic on an adjacent frequency channel (i.e., adjacent channel interference). If co-channel and/or adjacent channel interference is substantial, the signal quality associated with the radio signal may be severely impacted, so much so, that the connection may be dropped. In theory, frequency hopping, through the introduction of interference diversity, spreads the co-channel and adjacent channel interference amongst numerous end-users, such that the co-channel and adjacent channel interference experienced by any one particular end-user is diversified. The overall effect is to raise signal quality levels across the network, thereby improving overall system performance.
In accordance with typical frequency hopping schemes, each telecommunications connection, or for the purpose of simplicity, each end-user (e.g., cellular telephone or other mobile station) is assigned a frequency hopping sequence (FHS) at set-up (e.g., call set-up), where the FHS defines the sequence of frequencies over which the corresponding signal will be transmitted and received. Each FHS, in turn, consists of two basic parameters: a hopping sequence number (HSN) and a frequency offset (FO). The GSM standard defines the HSN as an integer number that may range from 0 through 63, where each HSN value 0 . . . 63 represents a different sequence of frequencies. To simplify the following discussion, specific HSNs are identified by a subscripted integer value. Thus, for example, an HSN equal to 63 will be represented as HSN63. In contrast, the FO is an integer number that ranges from 0 through N−1, where N represents the number of frequencies that are available for frequency hopping, and where FO represents a displacement in the frequency domain from a corresponding HSN. In the GSM, FO is called the Mobile Allocation Index Offset (MAIO). Again, to simplify the following discussion, specific MAIOs are identified herein by a subscripted integer value. For example, a MAIO equal to 20, would be represented as MAIO20. In general, each cell is assigned an HSN, where one skilled in the art will understand that a cell represents a geographical area in which a base station, or base transceiver station, communications with end-users (e.g., mobile stations) over a given set of RFCHs. In contrast, each end-user is assigned a MAIO. Thus, if an end-user has been assigned MAIO1, and the end-user is operating in a cell where the corresponding base station has been assigned HSN1, the FHS associated with the end-user is defined, in whole or in-part, by HSN1, and MAIO1.
FIG. 1 illustrates the concept of HSNs and MAIOs. More specifically, FIG. 1 depicts three cells A, B and C associated with an exemplary cellular telecommunication system, such as the GSM. In addition, FIG. 1 indicates that cells A, B and C have been assigned HSN0, HSN4 and HSN18, respectively. If, for example, there are 16 frequencies, f0 through f15, available for frequency hopping, in each of cells A, B and C, the sequence of frequencies associated with HSN0 might include the following sequence: f1, f5, f12, f9, f6, f7, f0, f15, f12, f4, f1. FIG. 1 also indicates that there are two end-users (e.g., two cellular telephones or other mobile stations) operating in cell A. The first mobile station (MS1) has been assigned MAIO4, while the second mobile station (MS2) has been assigned MAIO0. Accordingly, the FHS for MS1 is defined, in whole or in-part, by HSN0 and MAIO4. Assuming MAIO4 represents a frequency offset of +4 frequency channels, MS1 will communicate over a sequence of frequencies that includes the following sequence: f5, f9, f0, f13, f10, f11, f4, f3, f0, f8, f5. It then follows that MS2, which has been assigned MAIO0, will communicate over a sequence of frequencies that includes the following sequence: F1, F5, F12, F9, F6, F7, F0, F15, F12, F4, F1.
In order to minimize co-channel interference (i.e., the interference between end-users in two different cells communicating over the same frequency channel at the same time), and to a lesser extent, adjacent channel interference (i.e., the interference between end-users communicating over adjacent frequency channels at the same time), one skilled in the art will readily appreciate the desire to widely distribute FHSs that have the potential to cause co-channel interference and/or adjacent channel interference. By widely distributing these FHSs, the physical distance between two end-users operating on the same frequency channel, or on adjacent frequency channels, increases, which in turn, tends to reduce the adverse effects of co-channel and/or adjacent channel interference. However, very few FHS allocation strategies have been devised. In fact, the GSM standards (e.g., ETSI TS 100 908) do not define any such strategy. Accordingly, the allocation of FHSs is typically achieved in a somewhat ad-hoc or random manner. Thus, while frequency hopping techniques help to improve signal quality levels and overall network performance, the signal quality levels achieved are far from optimal.
To overcome the performance deficiencies associated with the ad-hoc or random FHS allocation strategy associated with GSM, International Patent Publication No. WO 96/02980, entitled “Channel Hopping in a Radio Communication System, ”describes a measurement-based FHS allocation method. More particularly, this publication describes a method that involves measuring the performance associated with each FHS. FHSs that exhibit greater measures of performance may be assigned to existing calls to replace FHSs that exhibit lesser measures of performance.
Neither the ad-hoc approach associated with the GSM, nor the measurement-based approach described in International Patent Publication No. WO 96/02980, directly rely on the level of interaction, i.e., the number of “collisions”, between FHSs. As the present invention is primarily focused on minimizing co-channel interference, the term “collision”, in the context of the present invention, refers to the situation where two mobile stations hopping over two different FHSs simultaneously hop to the same frequency. However, it will be understood that the present invention could be extended to include provisions for minimizing adjacent channel interference, whereby the term “collision” might refer to the situation where two mobile stations, hopping over two different FHSs, simultaneously hop to adjacent frequencies.
FIG. 2 illustrates the interaction, in terms of collision rate, over twenty 30-second time intervals, between a first exemplary FHS and two different FHSs. More specifically, FIG. 2 shows the number of collisions occurring between a first FHS, defined by HSN1 and MAIO0, and a second FHS, defined by HSN33 and MAIO8. FIG. 2 also shows the number of collisions occurring between the first FHS and a third FHS, defined by HSN33 and MAIO0. As indicated, the level of interaction, that is, the number of collisions, associated with the former FHS pair is significantly large as compared to the level of interaction associated with the latter.
Given the fact that the level of interaction between FHSs can vary significantly, as illustrated in FIG. 2, and given the fact that greater levels of interaction can adversely affect the signal quality of a call, it would be highly desirable to provide a frequency hopping technique, for use in telecommunications systems, such as cellular radio telecommunications systems, that takes the level of interaction between FHSs directly into account when allocating and assigning FHSs. Such a technique would be particularly desirable for use in networks with a tight frequency reuse plan (e.g., 1-reuse).